The present invention relates to a method and system for monitoring fish population data, such as the average length, depth and number density of fish with swim bladders in littoral seas. More particularly, the present invention relates to a fixed-source/fixed-receiver system that measures transmission loss over a large band of frequencies in such littoral environments.
Historically, the number of densities of fish have been measured with ship mounted sonar source-receiver systems that typically operated at 38 kHz. At these frequencies sonar source-receiver systems are not sensitive to fish length. To determine distributions of fish length and type, fish have to be sampled using nets, which are towed from ships. Fusing of the two types of data permits estimation of the number density versus fish length and type. These kinds of measurements are plagued by uncertainties, including those which are caused by ship avoidance behavior exhibited by fish, and by other effects. These effects are so complex that quantitative analysis of the uncertainties in the resultant estimates of number densities is extremely difficult. As a result, in general, quantitative measurements are not taken. The cost of such measurements is expensive, as it is driven by the cost of ship time at sea.
Consequently, measurement of the temporal evolution of number densities is generally not practical. Furthermore, such measurements are not synoptic. During the course of a measurement, which may require several days to complete, the spatial concentrations of fish may change; consequently, such measurements do not provide useful information about spatial distributions.
Transmission loss measurements made over a long duration over large frequency bands between fixed sources and receivers provide information about the size and number density of pelagic fish in littoral environments. The frequency of absorption lines is related to the size of swim bladders and the depth of absorbing layers, and the magnitude of the absorption coefficient is related to the number density (commonly referred to as biomass by fisheries scientists).
Absorption coefficients associated with layers of fish may be derived from transmission loss measurements over a wide band of frequencies by comparing measurements with computations of transmission loss with theoretical models of sound propagati on that include absorption layers that represent layers of fish. These models require knowledge of the sound speed profile between the source and the receiver, the depth of the bottom and the physical properties of the bottom. Sound speed profiles can be derived from detailed temperature versus depth measurements, and limited salinity versus depth measurements at the source and receiver. The bottom depth can be measured with an echo sounder. The physical properties of the bottom can be derived collection and storage system. U.S. Pat. No. 3,982,222 (Urick) discloses a string of hydrophone units attached to a cable held in a substantially vertical position by a floating buoy and an anchor. U.S. Pat. No. 3,372,368 (Dale) discloses a string of hydrophone units maintained in a vertical line array by a truss arrangement. The vertical line array is held in a substantially vertical position by a free floating surface station.
In another prior art approach for monitoring fish population data, a series of hydrophones have been cabled to the shore. While this approach serves its intended purpose, it is prohibitively expensive and is site specific.
In accordance with the invention, an improved acoustic measurement method and system are provided for determining fish population densities and other data regarding fish and like aquatic creatures. The invention is substantially less expensive to make and use than prior systems such as those employing hydrophones cabled to shore and provides additional advantages as well. One important aspect of the invention is the use of a continuous wave (CW) source to generate CW signals over a range of frequencies providing a high signal-to-noise ratio when detecting fish. A further important feature concerns the ability to readily and effectively deploy and retrieve the sources and receivers. Another key feature of the invention is that the receiver-source system is autonomous and requires no additional monitoring once the system is deployed. Because of these features, the invention can be of particular value to commercial fishermen, who have legal restrictions with respect to the different species of fish that they catch, as well as to commercial manufacturers of sonar equipment, who can use the invention to calibrate the effectiveness of their equipment in different littoral environments.
In accordance with one aspect of the invention, a method is provided for determining data with respect to fish populations in shallow water environments, the method comprising: deploying at least one acoustic source unit and at least one acoustic receiver unit at first depths preferably near the bottom and below the thermocline (if a thermocline is present) and using the at least one receiver unit to receive and record first acoustical transmissions from the at least one source unit at these first depths; and deploying the at least one source unit and the at least one receiver unit at second, higher depths preferably near the surface and above the thermocline (if a thermocline is present) and using the at least one receiver unit to receive and record second acoustical transmissions from the at least one source unit at these second depths.
Preferably, each of the at least one source unit and the at least one receiver unit is, when fully deployed, anchored by an anchor to the bottom and is supported on a respective cable connected to a buoyancy unit and to the anchor, and each of the at least one source unit and the at least one receiver unit is positioned at the first depths by releasing the cable so that a first predetermined length of the cable is payed out as the buoyancy unit rise in the water. Similarly, the units are positioned at the second depths by releasing the cable so that a second, longer predetermined length of the cable is payed out.
Advantageously, each of the units is released from its respective anchor after receiving and recording of the first and second transmissions is completed so that the units can rise to the water surface for recovery.
In an important implementation, the at least one acoustic source transmits for twelve hours near the bottom (below the thermocline) and for twelve hours near the surface (above the thermocline) so as to better track the movement of the fish over a full day.
In accordance with a further aspect of the invention, a method is provided for obtaining acoustic data for use in determining fish population in shallow waters, the method comprising the steps of: deploying, in a shallow water site having a bottom, an acoustic source comprising a first buoyancy unit, a first anchor, at least one acoustic transmitter, and a first cable connecting said first buoyancy unit to the first anchor and supporting the transmitter; deploying at least one acoustic receiver comprising a second buoyancy unit, at least one hydrophone, a second anchor, and a second cable connecting the second buoyancy unit to the second anchor and supporting the at least one hydrophone; separately releasing said first and second cables so that said first and said second buoyancy units rise in the water so that, with the source and receiver anchored to the bottom by the respective first and second anchors, said transmitter and the acoustic transmitter preferably comprises continuous wave broadband transmitter. Advantageously, the broadband transmitter transmits at frequencies between 0.2 and 10 kHz. In a beneficial implementation, the transmitter transmits an acoustic signal having an intensity level of at least 160 dB (preferably 170 dB).
Preferably, the source further comprises a control unit supported on said cable for supplying power to the transmitter and for controlling the operation of the transmitter.
The control unit advantageously includes a computer programmed to control start and stop times of transmissions by the transmitter.
The deployable source preferably includes a depth sensor mounted on the cable.
In accordance with a further aspect of the invention, a deployable receiver is provided for use in determining the fish population in a water environment at which the receiver is deployed, the receiver comprising: a buoyancy unit; at least one acoustic hydrophone means for receiving and recording acoustic signals for use in measuring fish population data; an anchor; a deployable cable, connected at one end to the buoyancy unit and at the opposite end to the anchor, for supporting the hydrophone at an intermediate point along the length of the cable such that when the cable is deployed, the buoyancy unit rises and the hydrophone is suspended by the cable between the buoyancy unit and the anchor, and an anchor release means for releasably connecting the cable to the anchor.
Advantageously, the anchor release means of the receiver (and of the source) comprises both an acoustic release unit and a timed release unit.
Preferably, the at least one hydrophone means includes four serially arranged hydrophones. The at least one hydrophone means comprises at least one and preferably four hydrophones for receiving the acoustic signals, signal conditioning means for conditioning these signals, an analog-to-digital converter for converting the signals into digital signals, a computer for processing the digital signals and a memory for storing the digital signals.
As with the source, the receiver preferably further comprises a depth sensor supported on said cable.
Further features and advantages of the present invention will be set forth in, or apparent from, the detailed description of preferred embodiments thereof which follows.